Numerous different constructions and geometries of antenna devices for electronic product anti-theft systems are known and in use. Reference can e.g. be made to: U.S. Pat. No. 4,251,808, U.S. Pat. No. 4,135,183, U.S. Pat. No. 4,243,980, U.S. Pat. No. 4,751,516, U.S. Pat. No. 872,018, U.S. Pat. No. 4,260,990, U.S. Pat. No. 4,016,553, U.S. Pat. No. 4,720,701, EP-A2-414,628, FR-763,681. Reference can also be made to U.S. Pat. No. 2,597,518, which describes a receiving antenna with several partial loops twisted with respect to one another by 180.degree..
FIG. 1 shows an antenna device, such as is frequently used at present. It has in series three partial loops 1,2 and 3 in each case reciprocally twisted by 180.degree.. The twisting of the partial loops against one another is for far field cancellation purposes. The ohmic resistors 7,8 provided at the intersection points 5 and 6 serve to somewhat "blur" the antenna characteristics and therefore avoid detection dead zones. FIG. 2 shows a field intensity distribution produced by this antenna device at a distance of 30 cm above the xy plane of the three partial loops. In accordance with the three partial loops there are three maxima, the field intensity decreasing from the first to the third partial loop. This undesired intensity decrease is caused by the current intensity decreasing between the individual loops. The overall inductance of the antenna device also determines or limits the antenna current, in addition to the voltage applied to the antenna terminals and this must be as large as possible. In modern systems there is a voltage of approximately 50 V at the antenna terminals 12. An increase in the antenna current by increasing said voltage is only possible with considerable effort and expenditure.